Keyholing at 1000 yards

C

Chuck Novice

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I am a new member and this is my second post. However, thanks in advance for the responses.

For the first time, I was able to shoot my Remington 700, 308, 26” heavy barrel at 1000 yards and at 875 yards. I set up a 48” X 48” target board first at 997 yards. I used my Wal-mart Nikon 550 range finder in a three stage relay to verify the distance because it is only reliable to 400 yards on all targets.

I fired 4 rounds at 997 yards, made a scope adjustment and fired 3 more rounds, and then checked the target. All 7 rounds impacted the target board, but none hit the 11” X 17” target paper in the center of the board. All the bullet holes showed signs of keyholing; some were much worse than others. I moved the target board to 875 yards and made a scope adjustment for that range and fired three more times. When I checked the target board, I found that all three rounds showed keyholing. However, the keyholing was not nearly as severe as at 1000 yards; again all three hit the target board, but none hit the 11” X 17” target paper. I went back to my shooting position, made another scope adjustment of one MOA up and one MOA to the left and fired three more times. Back at the target board again, I found all three had hit the target paper making distinctive keyhole markings in the paper. The group was approximately 14” high and 5” wide (I didn’t have a yardstick so I couldn’t make an exact determination of the group size.) This load of 168 Hornady Match BTHP over 45 gr. of Varget at 2700 FPS has shot less than 1” at 100 yards. Is it typical of a 30 cal 168 grain bullet fired at 2700 FPS to keyhole at those ranges?
 
I don't know about the Hornady 168 HPBT. It would be true for the Sierra 168 International Match. If the Hornady has the same bottail angle & size, it probably is true for the Hornady as well.

This is a case of dynamic instability of the bullet. We do not know for certain the cause of dynamic instability. We do know it has nothing to do with the rate of twist. Bob McCoy had a long article on the 168 Sierra International Match in one of the Precision Shooting annuals, worth reading.

The short answer for you is, get a different bullet. Borrow from the Palma shooters, who have done a lot of the work -- either the 155, as used for international competition, or the 175 Sierra, 185 Berger, etc.

See esp.

http://riflemansjournal.blogspot.com/p/articles-index.html

for some help.

Finally, for 1,000 yard benchrest, the .308 will rank way down at the bottom for effective chamberings, but I suspect 1K benchrest is not really your agenda.
 
Just curious, what is your twist, what elevation are you shooting at and at what temperature? 2700 fps muzzle velocity is right on the ragged edge for 1,000 yds under normal conditions for a 168 gr bullet. - nhk
 
The 168 gr Hornady has a short 13 degree boattail, the same as the 168 grain Sierra which was designed for 300 meter International shooting. Both of them, along with the Nosler 168 grain and the Lapua 170 grain are excellant out to 600 yards in the normal 2600 fps Match loads, but begin to become unstable beyond that distance as they become trans-sonic. But a velocity of 2700 fps should keep them pointy end first at 875 yards, and with the correct twist, out to 1000 yards, as nhk said. In the 1980s the USN and USMC shot the 168 grain Sierra at 1000 yards and won a lot of competitions but they were shooting hot loads (2700 fps +).

I'd check the twist of your barrel.

As an alternate to the 155 grain bullets you might try the 175 grain SMK, assuming you have the right twist to stabilize them. The current US sniper/match cartridge (M118LR) uses that bullet.

Ray
 
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BTW Chuck, this is going to get me into hot water but I wouldn't recommend the 155 grain bullet if you intend to do a lot of long range shooting. That bullet is the product of the Palma international rules. I think that most Palma shooters would prefer a heavier bullet but they are bound by the rules, and changing them (the rules) is slower than a second coming. Use the 175 grain SMK.

JMHO

Ray
 
Ray, by itself, rate of twist usually has no effect on keyholing, at least, at any range beyond 100 yards. If your twist isn't fast enough, you'll get pie-plate size groups at 200, and I suppose there will be some evidence of yaw, but not keyholing.

There have been some reports (Litz, Salazar) that an Sg of less than 1.4 can cause groups to open up at longer ranges, but again, this isn't keyholing -- See German Salazar's blog. And to counter that, the 187 FB I shoot at 1,000 yards was perfectly point-on, with small groups, using a 13.5 twist barrel. (Pushed a tad harder than a .308, of course.)

That was one of the things about the 187 FB -- Even with Tooley's 14-twist barrel, bullet holes showed no yaw, while Dave was winning a bunch of wood. That 14-twist barrel did give the aforementioned pie-plate size groups at 200 with the 200 grain SMG.

But yes, in order to give him any real help, we do need to know the twist rate.

Edit -- Ray, we posted simultaneously -- in an email, Jim Hardy mentioned that the 155 is required only for international competition. At the home-level game, even at Perry, there is no limit on the bullet. The 185 Berger (the LRBT, not the VLD) has done well, as has the 175 SMK. There are also some newer 155s that seem to shoo pretty good -- the BIB, and the new Sierra. Guess I'd trust the BIB with J4 jackets to be better over time than the Sierras, based on past Sierra performance.
 
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Charles

I didn't mean to imply that a wrong twist caused keyholing. If I did, I take it back. ;)

You are right, the 168 grain SMK is not a good long range bullet but it wasn't designed to be shot more than 300 yards, at a piddly 2250 fps at that. The Army and the USN and USMC proved that it's range can be extended all the way to 1000 yards with more speed but that's not really a good solution.

Yes, it's those pesky International Palma rules that call for the 155 grain max weight bullet. But Palma has a 135 year tradition that's not likely to be broken anytime soon.

Ray
 
We're sort of hijacking his thread, but yes, in days past, the 168 Sierra International Match was the U.S.'s answer for 300 meter ISU competition. That's a tough game, and the success of the bullet at that distance attests to it's quality (for that distance) -- same as the 6.5x55 for the Swedish teams. Now, of course, there are better choices for 300 meter ISU. I have an Italian friend who shoots that sport, and he says most of the European shooters go with the 6mm BR and 105-grain bullets.

I'm currently planning a smallish .30 for 600 and 1,000 yards. The case will be very close to a 30/284 or .30/06, and bullets considered will be the 185 Berger, 187 BIB, and a "secret" 210, very close to the Berger LRBR 210. Barrel will be an 11-twist, for an Sg of 1.4 with those 210s. That chambering is bigger than a .308, of course.
 
I'm out of my league here, but I tried 1,000 yd F-Class with the 168 gr Hornady A-Max at 2800 fps (and 6,500 feet of elevation and 80+ degrees) out of a .308, 26" 1:10 twist barrel (Sg = 2.06, BC = .475), and had no problems with that bullet, which has a polymer tip. I'd be interested in seeing a picture of the bullet holes. - nhk
 
nhk, I don't have even pictures of the bullets to hand, but it is *probably* the boattail of the 168 SMK that is the culprit for it's dynamic instability in the lower portion of the transonic region. If the Hornady HPBT is of the same design, it too will likely suffer the same fate. The A-Max is likely an entirely different design.
 
The twist in this rifle barrel is 1/12". The keyholing at 1000 yards was significant, at the 875 range the bullet holes looked more like a yawing bullet than a true keyhole. 1000 yd benchrest is not my goal here; one religion is enough for this man. Someone asked weather and altitude: 3000 feet above sea level, low humidity, 63 F, 4 mph wind at 2: o’clock to the shooter. I loaded up some 175 SMKs to try next time.
Next time may be this week may be next month, I will let you know.
What are the lower limits at which a bullet goes into "dynamic instability"? Neat phrase, I think I will throw it out at a party to impress some unsuspecting soul.
My ballistic program says that at 875 yds this 168 Hornady is traveling at 1440 fps and 1304 at 1000 yds. My casual recollection is that the speed of sound is about 1200 fps and slower at higher elevations and that a bullet is stable above the speed of sound.

Chuck Novice
 
Chuck,

There is a region just above the speed of sound (and possibly below, but irrelevant for our purposes) referred to as the 'trans-sonic region'. Basically as the bullet is slowing down approaching the speed of sound, it starts experiencing increased turbulence as it gets closer. It isn't just one side of the sound barrier a bullet is stable and the other side it's not, but that increased turbulence that causes some bullets to start to become unstable and yaw and keyhole through the target. Some bullet designs do better at making it through this transition area than others; the relatively steep 13 degree boat tail on the S168MK and its clones (in way of contrast, the B168VLD has a more normal - for long range - 9 degree boat tail) is usually cited as the cause for its poor behavior at this stage. I don't know that I've ever seen a hard number nailed down for the start of the trans-sonic region, but 1300fps is normaly a good point to stay *above* if you can help it - but that can be tough with a .308 Win!
 
"buffeting" would make any ballistician (of which I am not one) squirm. A bullet is dynamically unstable when the angle of attack reaches a certain point and the slow arm of the ptich/yaw cycle does not remain in equilibrium, but grows. This is not true for all, or even most, bullets.

I couldn't find McCoy's Precision Shooting article on the web. Here is an earlier analysis performed for the government. It's ours, we paid for it.

http://www.dtic.mil/cgi-bin/GetTRDoc?Location=U2&doc=GetTRDoc.pdf&AD=ADA205633

McCoy obviously did the further analysis he recommended.

If you want to search for more information, look for "dynamic instability" or especially "magnus moment". On the other hand, you could just try some of the other bullets mentioned . . .
 
German: specifically, The 1996 Precision Shooting Annual. It looks like PS has the copyright, which I find surprising.

The nice thing about the McCoy article is it is based on empirical data, which is too expensive for most of us to get.
 
I asked the question didn't I. (What are the lower limits at which a bullet goes into "dynamic instability"?) Now that it is too windy to shoot where I can shoot 1000 yds (20 to 30 gusting to 45) I guess I must read some of those articles. That is some heavy reading, glad there are some pics in that 73 pages of http://www.dtic.mil/cgi-bin/GetTRDoc?Location=U2&doc=GetTRDoc.pdf&AD=ADA205633
Ya’ll are trying to educate me on this matter aren’t ya.
Chuck soon the UnNovice
 
I know from previous discussions that I am not on the same page as some others with my definition of keyhole'ing. If someone would kindly clarify what part of my thinking is wrong, I'd appreciate it. Here's the way I see it with examples.

1: Sierra 55Gr .224 plastic tipped bullet in 14 twist factory 22 Remington. Shoot that bullet at 100 yards, it makes a hole resembling a profile of the bullet in the target paper but at 50 yards, the holes are mostly round. Past 100, you can't hit a truck. Increase twist to the 12 recommended by Sierra and the bullet will arrive point on.

2: Shooters shooting at 1000 yards have bullets arriving at the target making a complete profile hole as they pass through. Again, I call this keyhole'ing. Change barrels and increase the rate of twist for that same bullet, and it will work just fine. It's made a rather long trip and still hit the target, and at most intermediate ranges, this bullet would group almost acceptably to some.

I have observed these situations with my own rifles. I say the bullets are keyhole'ing and that it is caused by not enough rate of twist. Now, perhaps semantically, there is something someone doesn't like about the terms I'm using. Perhaps, there is a flaw in the logic. As I have seen it, these are my observations. What am I missing? What is wrong with the way I say this?

>>>>>>>>>Edit to add
I also have said that these bullets are unstable. That has been disputed in the past. Are these bullets that are traveling sideways and tumbling, unstable or not?
 
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Phil, you have to take it one thing at a time. Suppose you have a bullet that keyholes at 1,000 yards -- like the 168 SMK. OK, you've shot it from a 12-twist barrel. You increase the rate of twist to a 1-10, and it still keyholes. Again to 1-9, and it still keyholes. Conclusion: it ain't the twist. Then you pump the sucker out of a 300 Bear, and with the old 12-twist barrel, it doesn't tumble. Conclusion: terminal velocity must be at play here.

On the other hand, just because a bullet is dynamically unstable (usually shows up in the transonic region) doesn't mean it can't be both gyroscopically unstable, with a certain twist rate, as well. Witness your examples.

See

http://www.nennstiel-ruprecht.de/bullfly/dynacond.htm#header

Which I'll allow isn't terribly helpful, because you need empirical data for useful calculations.

In general, gyroscopic stability is determined by rate of twist. Your .223 with the long bullets is a perfect example of gyroscopic instability. Pie-plate size groups at short range. Moreover, Sg tends to increase as a bullet files downrange, regardless of the velocity. Dynamic stability, Sd, is quite different, and minimally affected by rate of twist. You won't find Sd numbers on the drag-twist page!
 
Thanks Charles,

Actually, that did help quite a bit. I get the picture without plugging numbers in and the descriptions below pretty much tell the story.

I see then that what you're saying is that the two conditions are from differing problems. The 55gn'er is gyroscopically unstable at the muzzle, and the 210 @1k simply ran out of gas. And, the saying that the 210's were not stable is from a Dynamically or Gyroscopically stable semantic viewpoint, even if they can't make the trip through the transonic region.
 
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